The problem occurs after the Avast! installer performs a re-boot, which procedes normally up to the point of displaying the wallpaper but never displays the taskbar or any desktop icons. Cntl-alt-delete shows explorer running, but fails to stop it when requested and fails to shutdown when asked, leaving the only option to pull the plug.

SafeMode uninstall Avast! cures the boot problem, and a clean install of Avast! works normally, together with several ZA requests for permission to allow AV access to the internet etc.

This is the second occurrence of this problem, so where does the incompatibility lie ? Well obviously in Avast! I hear you say. Somehow I doubt it, because ZA does not ask about the new AV programs during the update (they are not being run at this point).

I think ZA is trying to ask for their permissions before Windows can display the requests, eg: too early in the boot process.